Electro-hydraulic transducer amplifier with a plurality of control inputs, and associated methods

ABSTRACT

A hydraulic transducer comprises in a single unit and in a single casing an electromagnetic motor with a magnetic circuit and a control circuit each having a respective plurality of coils for receiving first and second series of signals. A magnetic blade rotates in the air gap of the magnetic circuit under a torque proportional to the product of the weighted algebraic sums of the respective series of signals. Coupled to the magnetic blade is a second blade oscillating in front of one or more hydraulic jets which operate a servo-distributor.

United States Patent 11 1 1111 3,878,866

Lucien Apr. 22, 1975 [5 ELECTRO-HYDRAULIC TRANSDUCER 2.587.983 3/1952 Durand 335/231) AMPLIFIER WITH A PLURALITY OF $920.83) 7/196: Ra) 1.12/83 CONTROL INPUTS. AND ASSOCIATED 512; l 7 3 METHODS 3.225.346 12/1965 Buddenhagcn 137/112 x {75] Inventor: Rene Lucien. Seine. France FOREIGN PATENTS OR APPLICATIONS [73] Assignee: Recherches Etudes Production W9 1/1964 Japan R.E.P.. Paris. France [22] Ffled: May I 1972 Prl'mm l liruminer-Arnold Rosenthal [2 l] APpl. No.: Aflyrn Agenh or v v odin Schwanz &

Related U.S. Application Data N556" [63] Continuation-impart of Scr. No. 68.557. Aug. 3i. l970. abandoned. which is a continuation of Scr. No. 548.30l. May 6. I966. abandoned. [57] ABSTRACT A hydraulic transducer comprises in a single unit and l l Foreign Application Priority Dam in a single casing an electromagnetic motor with a May 7. 1966 France 6618268 magnetic circuit and a control circuit each having a respective plurality of coils for receiving first and sec- [52] US. Cl. 137/625.6l; 137/83; 137/625.64; ond series of signals. A magnetic blade rotates in the 251/30; 335/268 air gap of the magnetic circuit under a torque propor- [51] Int. Cl. Flfik 31/02; GUSd l6/20 tional to the product of the weighted algebraic sums of [58] Field of Search..... l37/625.62. 625.6]. 625.64, the respective series of signals. Coupled to the mag 137/82. 83; 3l7/l55.5; 335/266. 268 netic blade is a second blade oscillating in front of one or more hydraulic jets which operate a servo [56] References Cited distributor.

UNITED STATES PATENTS 2.227.351 12 1940 Klein 335 2511 x 6 Drawmg F'gum a? fl-ga {is a 7 ..g 5/ 2 \l l i 1 *7 l L $61:

I /34 3/ 33 gm PHENTEB APR2 2 595 saw 5 D? 5 FIGS? 2 2 2 2 2 I 1|! m. M l I! H MI HM? a... f! I ,ll a 1. (Ill. Ill|| FIG 10 ELECTRO-HYDRAULIC TRANSDUCER AMPLIFIER WITH A PLURALITY OF CONTROL INPUTS. AND ASSOCIATED METHODS CROSS RELATED APPLICATION This Application is a continuationin part of Application Ser. No. 68.557 filed Aug. 31. 1970 now abandoned which in turn is a streamlined continuation of my earlier Application Ser. No. 548.3Ul filed May 6. 1966 now abandoned which claimed the priority of my French Application filed May 7. I965.

BRIEF SUMMARY OF THE INVENTION The present invention relates to apparatus capable of supplying. at substantial power. a hydraulic output pa rameter (pressure of flow-rate of a fluid received from a source of supply) which is a function of the product of the weighted algebraic sums of a number of electrical signals applied to its inputs at an extremely-low level of power.

An apparatus of this kind finds a special application when it is desired to obtain. as a function of a number of electrical information signals. a pressure or a parameter which can be obtained from a pressure (force. torque. work. acceleration. etc.). or a flow-rate. or a parameter which can be obtained from a flowrate (linear speed. angular speed. etc.). for example. in order to obtain a direct control. a servo-control or a regulation.

In theory. it is easy to design such an apparatus; it is only necessary to associate an amplifier. electronic. magnetic or other type. with a plurality of control inputs which are capable of multiplying together the signals applied to its inputs. and an electrohydraulic servo-distributor. Such a unit would however be fairly complicated. heavy. bulky and costly both to buy and to maintain.

In consequence. the invention has for its objects the provision of an apparatus fo this kind which is relatively much simpler. very much less bulky and less expensive than the abovementioned combination.

The invention has also for its object the provision of an apparatus of this kind which has great reliability.

Finally. the invention has for its object the provision of an apparatus of the kind referred to which has a high amplification ratio and high dynamic characteristics which render it suitable to form part ofa servo-control line.

The apparatus according to the invention comprises an electro-magnetic motor actuating a blade which oscillates in front of one or more hydraulic jets. this subassembly corresponding generally. but with various adaptations and various improvements to the apparatus described in U.S. Pat. No. 3.054.416 of Sept. l8. I962 and in the US. Pat. Nos. 164.094 of .Ian. 3 1962. now U.S. Pat. No. 3.772.149. and 433.758 of Feb. l8. I965 in the name of the present applicant.

By reason ofthe said improvements and adaptations. the description which follows below of the apparatus in accordance with the invention will make no further reference to the above-mentioned patents and patent applications.

Briefly. the apparatus according to the invention comprises. in a single unit and in a single casing. an electromagnetic motor with a number of control windings mounted on a magnetic circuit. in the air-gaps of which oscillates a magnetic blade. a rigid shaft coupling with a fluid-tight torsion tube between this magnetic blade and a blade oscillating in front of one or more hydraulic jets. and a servo-distributor operated by the said jet or jets.

The invention provides that the electro-magnetic motor can receive electrical signals. of direct or rectified current. modulated in amplitude. or of alternating currents of the same frequency. modulated in ampli tude or in phase. The invention also provides that the electro-magnetic motor may carry out the algebraic addition of the input signals and the multiplication of their sums; and finally. the servo-distributor. supplied from a hydraulic source. can deliver a hydraulic pressure or a flow-rate which is a function of the input signals.

The apparatus according to the invention has an overall size substantially the same as that of the servodistributor alone. it is not substantially complicated. and in consequence the invention provides an apparatus of small bulk. low weight and price. and the reduction of the number ofits components gives it great reliabilitv.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described with reference to the accompanying drawings. given by way of nonlimitative examples. In these drawings:

FIG. I is an elevation view in section ofthe apparatus in accordance with the invention;

FIG. 2 is a section taken along the line ll II of FIG. 1:

FIG. 3 is a partial section taken along the line "I III of FIG. I;

FIG. 4 is a perspective view partly in section of the apparatus of FIG. I but with the control coils and arm ature placed above the operating coils instead of laterally thereof for the sake of clarity;

FIG. 5 is an explanatory circuit diagram of the apparatus using magneto-electrical analogy:

FIG. 6 is an elevation view in section of a further em bodiment according to the invention:

FIG. 7 is a block diagram showing a control by rectified signals of an apparatus according to the invention;

FIG. 8 is similar to FIG. 2 showing another motor of an apparatus according to the invention;

FIGS. 9 and I0 are similar to FIG. 2 showing two further motors according to the invention; and

FIGS. ll I3 are diagrammatic illustrations for establishing a mathematical proof of the operability of the circuit as a product of weighted algebraic sums of two series of signals.

DETAILED DESCRIPTION Referring now to FIGS. I 3. the apparatus according to the invention comprises. in a single casing I. an electro-magnetic motor 2 comprising a plurality of op erating or control coils 3a. 3b. 3c 3n. and a magnetic blade 4 oscillating in the air-gaps 23 of a magnetic circuit 2]. The magnetic blade 4 is rigidly coupled to the hydraulic blade 5 by a shaft 6; the shaft 6 is supported at its two extremities by two pins 7 and 8. of small diameter and small length. which. in turn. are mounted in the casing 1. In addition. a torsion tube 9 has the triple function of a support. a restoring spring and a fluid seal between the electromagnetic motor 2 and the chamber of the jet or jets 10. The oscillation of the hydraulic blade 5 in front of the jet or jets 10 results in variations of the pressure losses in the hydraulic circuits tnot shown in FIG. ll which control the move ments of a slide-valve ll of the servo-distributor 12. This distributor l2 derives its supply (arrow A) from a hydraulic source [3. and the movements of the slidevalve ll produce variations of pressure P or of flowrate Q at the outlet 14 of the apparatus according to the invention.

The motor 2 comprises a second magnetic circuit 22. The magnetic circuit 22 includes a second plurality of control coils 61 filg. .6i,,,. The magnetic blade 4 oscillates in the air-gaps 23 of magnetic circuit 2 under the action of the magnetic fluxes produced by the first and second sets of control coils. Referring to FlG. 4 which is more specific. therein it can be seen that end 4a of blade 4 travels in air-gaps 23a and 23h while end 411 travels in air-gaps 23c and 23d. The two halves of the magnetic circuits 21 are designated by numerals 21a and 21h.

FIG. 5 shows the magnetic circuits according to conventional electrical analogy. Therein coils 3a. 3b. 3c. 3d

. are shown as sources of power as are also the coils 61.. 61 The magnetic circuits Zlu. 21b and 22 are shown as electrical conductors. and the air-gaps 23a. 231 23c. 2311 are shown as electrical resistances.

The total flux produced by all the coils 3a. 3b. 3c. 31!. flows through blade 4 toward the end 4a and there splits into two haives: one half crosses the air-gap 2311 from bottom to top in FIG. 5 and follows circuit 210 from left to right. then crosses airgap 23c from top to bottom. and returns through end 41) of blade 4; the other halfofthe same flux crosses air-gap 2317 from top to bottom. follows circuit 2111 from left to right. crosses air-gap 23d from bottom to top. and rejoins the first half of the flux upon return to end 4b of blade 4.

The total flux produced by the coils 6] 61 etc. follows the magnetic circuit 22 and the magnetic circuit Zlu. where it splits into two halves: one halfcrosses air gap 230 from top to bottom. passes end 4a of blade 4 from top to bottom. crosses airgap 23b from top to bottom; the other half crosses air-gap 23c from top to bottom. passes end 41; of blade 4 from top to bottom. and crosses air-gap 231! from top to bottom. The two halves join up again in the magnetic circuit 21b and return through the magnetic circuit 22.

Thus. the flux produced by coils 3 flows longitudinally through the blade 4 within its depth. and the flux produced by coils 61 crosses the extremities 4a and 4b of the blade perpendicularly to its depth.

Moreover. the two fluxes cross the two diagonally opposite air-gaps (23b and 23c) in the same direction and the other two diagonally opposite air-gaps (23a and 23d) in opposite directions.

As seen in FIG. 9 the control coils 6] can be unsymmetrically placed at one side of magnetic circuit 22 whereas as shown in FIG. 10, the control coils can be symmetrically placed in magnetic circuit 22.

The movements of the hydraulic blade 5 in front of the jet l0 produce variations of pressure in the conduit 30 of this jet. supplied through the diaphragm 31 and the chamber 32 of the servodistributor 12 by the supply pressure A. The slide-valve ll of the servodistributor I2 is operated by the difference between the pressures in the chamber 33. in which the pressure is that of the conduit 30 upstream of the jet l0. and in the chamber 34, in which the pressure is the outlet pressure P at 14.

The slide-valve ll thus moves until it equalizes these two pressures.

FIG. 6 shows another apparatus according to the invention. this being different from that preceding in that. at its outlet. it controls a flow-rate and not a pressure. By means of the hydraulic blade 5. this apparatus acts to control two jets 10a and 10b. in which the differences of flow-rate produce differences of pressure in their conduits 30a and 301) by means of orifices 31a and 3H). These pressure differences are applied to the slidevalve 1] in the chambers 33a and 33h: the slide-valve ll is thereby displaced until it equalizes the ratio of flowrates in the outlets Q1 and O2 to the ratio of the flow-rates of the jets 10a and 10b.

With regard to the electro-magnetic motor 2. it will be noted that the magnetic flux generated in the mov ing blade 4 is the product of the fluxes produced by windings 3a. 3b. 3n and 61., 61 .61" of the separate coil groups. each of these windings receiving a respective control current. The number of turns of each winding makes it possible to give the desired weight to the current which is injected with respect to the other currents; in other words. it is the number of turns of each coil which permits the adjustment of the pressure P or flow-rate Q as a function of the input currents:

Flow rate Q B lk i k i nkn n) (K,l K l In other words. the pressure P or the flow-rate O is the product ofa factor (A or B) with the algebraic sum of the weighted elementary products k i, and K,,,l,,.. it will be noted that the direction in which the current is passed in the winding permits the determination of the sign of the given current with respect to the others. that is to say the introduction of minus signs in the above algebraic formula.

The factors A and B in the above formulas can be adjusted by acting on various constructional parameters of the apparatus. such as the dimensions of the slidevalve. section of jet section of air-gap of the magnetic circuit. It can be individually adjusted for each apparatus by acting on the flux density of the permanent magnet of magnetic-circuit 22, by means of the number or the size of the magnets. or by means of shunts of variable thickness shunting a more or less large part of the permanent flux. In addition. there may be provided a torsion tube which has a strength variable as a function of its torsion and by this means a variable coefficient A and B is obtained.

The above considerations are valid for direct current inputs. If one or more of the inputs are supplied with alternating current. it is only necessary as seen in FIG. 7 to rectify this current by means of a diode bridge 41 to re-establish the conditions of the preceding case. It is not however possible in that case to vary the direction of operation of the current which is fixed at the moment of the connection of the rectifier to the coil.

If all the inputs are alternating currents. it is then possible to eliminate the rectifiers and to vary the direction of the current by acting on its phase. As shown in FIG. 8 the permanent magnets can be entirely omitted. The coil 51 is supplied with an alternating current 1 having the same frequency as the input currents i i i...

These input currents must then be either substantially in phase or substantially in phase opposition with the current I. If at the output of the devices which have prepared them. the phase of one or more control currents does not comply with either of these conditions. it can be adjusted by passing through a dephasing device. for example an appropriate resistance-capacity circuit. It is however necessary that the phases of these signals should be substantially stable in time if it is desired to detect only the amplitude variations of these currents. 1

it is also possible to employ input currents which consist of alternating signals of fixed amplitude and variable phase with respect to the reference phase of the current I.

In the case where fixed phase amplitude-modulated signals are utilized. the different input currents will be additive or subtractive with respect to each other. depending on the direction of winding of the coils into which they are passed. and depending on their relative phases (to within 180). lt is therefore possible for the same input to reverse the direction of flow of the current passed, by reversing the phase of this latter by 180.

The same considerations as shown above with respect to the adjustment of the parameters A and B also apply in this case to the extent that instead of being adjusted by acting on the continuous flux generated by the magnets. they can be adjusted by the number of turns of the coil SI traversed by the value of this current.

ln order to avoid losses due to eddy currents. there will furthermore be an advantage in the case of control by alternating currents, to replace the solid magnetic circuits employed with direct current by laminated or sintered circuits. and this advantage will increase as the chosen working frequency increases.

Finally. again for the case of control by alternating currents it is desirable that the natural frequency of the mechanical system should be less than twice the frequency of the current employed. so as to constitute a low-pass mechanical filter eliminating the oscillations due to the fact that the force acting on the magnetic blade passes through Zero twice in each cycle.

lt will be observed that it is possible to carry out mul tiplication by acting on the value of the control fluxes whether this latter is direct or alternating.

In the case of operation on alternating current. it is only necessary to make the control current I variable. and this latter can of course be applied by a number of coils each fed with a respective current from a separate power source and constituting one series of input information signals, the other series of input information being the respective currents flows fed from separate power sources to the respective coils of the operating windings.

There will now be explained mathematically how there is obtained on the magnetic blade a torque proportional to the product of two algebraic sums ofelec tric signals. For this purpose. reference will be made to the simplified construction in FIG. 11 in which a single control coil and a single operating coil surround the blades The magnetic circuit shown in FIG. It has one control coil B of N turns and one operating coil 12 of :1 turns around the blade.

A, B, C. D, are four assumedly identical air-gaps formed between the pole members and the blade. each air gap being of surface area S and length 2.

The blade is assumed to remain stationary in the centered position.

k being a constant dependent on the units employed. In each air gap the flux will be:

v S lob-I Similarly flux d) generated in the blade by a current icirculating in coil b is distributed as shown in H0. ll (dotted line). its direction shown having been chosen arbitrarily.

Taking the same system of units as that used for computing flux d). the magnitude of this latter flux is given by the formula:

Am' Am In each air gap.

With the arbitrarily chosen directions for fluxes l and qb. the fluxes are added together in air gaps B and C, and substracted in air gaps A and D.

In gaps B and C. we have:

The induction in these air gaps will be:

In the gaps A and D (assuming Nl greater than ni) kS fl ln lb| Til |Ii) The induction in these air gaps will be:

k TtNl ni) The magnetic energy engaged in pulling the blade towards each pole is given by the formula:

,8 being the induction prevailing in the relevant air gap.

Here we have (see FIG l2) The expansion of which gives:

Here we see the product of the two balanced cur- 3 rents Similarly If r is the distance between the axis of the blade and the center ofthe air gaps. the torque acting to make the blade rotate is:

This torque is proportional to the product of the two input currents I and 1'. each of these currents being weighted by the number of turns in the coil through which it is passed.

This calculation based on the case of the blade remaining dead center. is still appreciably valid for angular displacements of the blade giving rise to a gap variation which is slight in comparison to the overall gap. equivalent to the range of the motor-torque. particularly when this is driving a hydraulic blade in front of one or more jets so as to vary the pressure at these latter Referring next to FIG 13, the same magnetic circuit as in FIG. 12 is shown but with p control coils. the numher of turns in each coil being respectively N,. N

. N each coil receiving respectively a current L. I.

The flux in each air gap due to the coils around the blade will be:

The resultant flux in each air gap and the corresponding induction. keeping to the same conventions of sign as before for the direction of the partial resultant fluxes. will be:

The corrresponding forces on the blade will be:

S FII 1 V Making K we obtain and similarly NI l The resultant forces will he:

and after expansion, this gives and similarly whereby the torque on The result is a torque proportional to the product of two algebraic summations. the terms of one of the series being provided by the current applied to each control coil. weighted by the number of turns of the relevant coil. and of an algebraic sign dependent on the direction of winding ofeach respective coil and the direction of flow of the current in such coil; while the terms of the other series are provided by the current applied to each coil surrounding the blade. weighted by the number of turns of the respective coil and of algebraic sign dependent on the direction of winding of the relevant coil and the direction of flow of the current in such coil.

What is claimed is:

l. A hydraulic transducer comprising in a single unit and in a single casing. an electromagnetic motor having a magnetic circuit with two air gaps and first and second pluralities of separate and independent. individual. control windings, means for applying a respective signal from a first and a second series of signals to a respective control winding of said first and second pluralities respectively. each winding having a number of turns proportional to the weight to be given its respective signal. said means for applying said signals comprising an alternating current source having phase or amplitude modulation of equal frequency for all windings. and means for converting each alternating current signal to a direct current signal modulated in amplitude. the mechanical system having a natural frequency which is less than twice the frequency of the input signals. said first plurality of windings producing a first magnetic flux which passes through said two air gaps. said second plurality of windings producing a second magnetic flux which is perpendicular to said first mag netic flux. one permanent magnet supplying an additional flux to one of said two fluxes. a magnetic blade rotatable in the two air gaps of said magnetic circuit under the combined influence of said two fluxes. said blade being angularly displaced in said air gaps by an amount which is a function of the product of the weighted algebraic sums of the two respective series of signals. a hydraulic blade. a rigid shaft coupling including a fluid-tight torsion tube connecting said magnetic blade and hydraulic blade. at least one hydraulic jet in front of which said hydraulic blade rotates. a servodistributor controlled by said jet and thereby by the posi tion of the hydraulic blade with respect thereto. a hydraulic source supplying said distributor. and an outlet orifice controlled by said distributor for delivery of hydraulic fluid at a flow rate related to the position of the hydraulic blade as influenced by the magnetic blade and hence as a measure of the product of the weighted algebraic sums respectively of said first and second series of signals.

2. A transducer as claimed in claim 1 wherein one half of said first flux passes through one of said air gaps at one extremity of said magnetic blade in a direction perpendicular to the blade. the other half of said first flux passing in the other of said air gaps at the other extremity of said magnetic blade in the same direction. said second flux passing longitudinally through the magnetic blade from one of said air gaps to the other.

3. A transducer as claimed in claim 1. further comprising two permanent magnets. each supplying an additional flux to one of said two fluxes.

4. A transducer as claimed in claim I. in which the transducer has an overall amplification factor which is a fraction ofthe dimensions of said distributor ofthe jet and ofthe air-gaps. and means for regulating the amplification factor by varying the magnetization of said windings S. A transducer as claimed in claim 1 comprising a common core for said first set of control windings. the latter being wound separately on said core and being individually supplied with a respective signal from a separate source.

6. A transducer as claimed in claim I. wherein said means for applying a respective signal to each control winding of the second set comprises a separate source for each such winding directly coupled thereto independently of all the other windings.

l =l= l 

1. A hydraulic transducer comprising in a single unit and in a single casing, an electromagnetic motor having a magnetic circuit with two air gaps and first and second pluralities of separate and independent, individual, control windings, means for applying a respective signal from a first and a second series of signals to a respective control winding of said first and second pluralities respectively, each winding having a number of turns proportional to the weight to be given its respective signal, said means for applying said signals comprising an alternating current source having phase or amplitude modulation of equal frequency for all windings, and means for converting each alternating current signal to a direct current signal modulated in amplitude, the mechanical system having a natural frequency which is less than twice the frequency of the input signals, said first plurality of windings producing a first magnetic flux which passes through said two air gaps, said second plurality of windings producing a second magnetic flux which is perpendicular to said first magnetic flux, one permanent magnet supplying an additional flux to one of said two fluxes, a magnetic blade rotatable in the two air gaps of said magnetic circuit under the combined influence of said two fluxes, said blade being angularly displaced in said air gaps by an amount which is a function of the product of the weighted algebraic sums of the two respective series of signals, a hydraulic blade, a rigid shaft coupling including a fluid-tight torsion tube connecting said magnetic blade and hydraulic blade, at least one hydraulic jet in front of which said hydraulic blade rotates, a servodistributor controlled by said jet and thereby by the position of the hydraulic blade with respect thereto, a hydraulic source supplying said distributor, and an outlet orifice controlled by said distributor for delivery of hydraulic fluid at a flow rate related to the position of the hydraulic blade as influenced by the magnetic blade and hence as a measure of the product of the weighted algebraic sums respectively of said first and second series of signals.
 1. A hydraulic transducer comprising in a single unit and in a single casing, an electromagnetic motor having a magnetic circuit with two air gaps and first and second pluralities of separate and independent, individual, control windings, means for applying a respective signal from a first and a second series of signals to a respective control winding of said first and second pluralities respectively, each winding having a number of turns proportional to the weight to be given its respective signal, said means for applying said signals comprising an alternating current source having phase or amplitude modulation of equal frequency for all windings, and means for converting each alternating current signal to a direct current signal modulated in amplitude, the mechanical system having a natural frequency which is less than twice the frequency of the input signals, said first plurality of windings producing a first magnetic flux which passes through said two air gaps, said second plurality of windings producing a second magnetic flux which is perpendicular to said first magnetic flux, one permanent magnet supplying an additional flux to one of said two fluxes, a magnetic blade rotatable in the two air gaps of said magnetic circuit under the combined influence of said two fluxes, said blade being angularly displaced in said air gaps by an amount which is a function of the product of the weighted algebraic sums of the two respective series of signals, a hydraulic blade, a rigid shaft coupling including a fluid-tight torsion tube connecting said magnetic blade and hydraulic blade, at least one hydraulic jet in front of which said hydraulic blade rotates, a servodistributor controlled by said jet and thereby by the position of the hydraulic blade with respect thereto, a hydraulic source supplying said distributor, and an outlet orifice controlled by said distributor for delivery of hydraulic fluid at a flow rate related to the position of the hydraulic blade as influenced by the magnetic blade and hence as a measure of the product of the weighted algebraic sums respectively of said first and second series of signals.
 2. A transducer as claimed in claim 1 wherein one half of said first flux passes through one of said air gaps at one extremity of said magnetic blade in a direction perpendicular to the blade, the other half of said first flux passing in the other of said air gaps at the other extremity of said magnetic blade in the same direction, said second flux passing longitudinally through the magnetic blade from one of said air gaps to the other.
 3. A transducer as claimed in claim 1, further comprising two permanent magnets, each supplying an additional flux to one of said two fluxes.
 4. A transducer as claimed in claim 1, in which the transducer has an overall amplification factor which is a fraction of the dimensions of said distributor of the jet and of the air-gaps, and means for regulating the amplification factor by varying the magnetization of said windings.
 5. A transducer as claimed in claim 1 comprising a common core for said first set of control windings, the latter being wound separately on said core and being individually supplied with a respective signal from a separate source. 